im unaware of diesels having 'fuel trim' because they dont look at lambda values of the exhaust, diesel fuelling is controlled via MAF sensor on the intake. diesels are a different animal to petrols because they are compression ignition rather than spark ignition, with spark ignition fuel trims are needed to compensate for the ignition process where as diesels inject fuel according to air mass and temp. you would want to look at 'injected fuel values' on the diesel and if one is wildly different to the rest you probably have an issue.
as for your boost issues... looks like you have a vac pipe off or possibly a dicky pressure transducer. follow all the thin rubber hoses from the vac pump on the passenger side of the engine on the end of the exhaust camshaft and check for any leaks and loose connections. *should* be pretty easy to find
'07 159 Lusso Saloon, 2.4 JTDm, Oltremare Blue.
'99 Fiat Coupe, 2.0 20v VIS, Portofino Blue, Full Leather, Mani-Back Longlife Custom S/S Exhaust.